Fail safe brake for rail type lifts

ABSTRACT

The load platform of a lift is carried by a runner mounted for vertical reciprocation along a hollow guide column. Within the column the runner and its load are suspended from a short length of a leaf chain that, in turn, is suspended from a powered vertically reciprocable wire rope. The leaf chain extends between a pair of cam pawls pivotally mounted on a brake body carried by the runner, with the pivot axis of the pawls oriented parallel to the pivot axes of the links of the chain. The pawls are biased in a direction to move out of a retracted position into an extended position wherein they engage one side of the column to jam the brake body against an opposite side of the column. The pawls have a contact pin confronting one side of the chain such that when the chain is under tension it bears on the contact pin and so maintains the pawls in retracted position. Upon a partial or total failure of the wire rope inducing a loss of tension in the leaf chain, the short, pivotally connected links of the chain are readily laterally deflectable and offer no significant resistance to movement of the spring biased pawls into the extended braking position.

BACKGROUND OF THE INVENTION

The present invention relates to safety devices for load elevators and,more particularly, automatically actuatable fail safe brake mechanismsfor platform lifts of the kind having hollow guide rails or columnsalong which the platform is raised and lowered.

Trucks are often fitted with powered load lifting and loweringappliances, commonly called lift gates, comprising a platform that ismovable between ground level and the bed of the vehicle, either at theside or at the tail end of the vehicle bed. In one type, the apparatusincludes a laterally spaced apart pair of vertically extending fixedguide columns each of which mounts a vertically elongate runnerassembly, the opposite sides of the load platform being connected to thelower ends of the pair of runner assemblies. Typically, the upper end ofeach runner assembly is interconnected, within the corresponding guidecolumn, to one end of a flexible wire rope, chain or cable that istrained around a sheave at the upper end of the guide column so that theplatform is, in effect, suspended from the cable or the like. The otherend of the flexible suspension element is usually interconnected to aunidirectionally acting hydraulic cylinder. Typically, the hydraulicsystem for the cylinder is such that when it is desired to raise theplatform from ground level to bed height, a pump is energized topressurize that side of the cylinder piston which will effect raising ofthe platform. Conversely, when it is desired to lower the platform frombed height the pump remains de-energized while a valve control isactuated to bleed the cylinder as the force of gravity lowers theplatform.

In a lift of this kind, as the platform is suspended by means of a cableor the like, the platform and its load, if any, may drop suddenly if oneof the cables supporting the platform breaks. In order to prevent theplatform from then falling to the ground, a safety device has previouslybeen devised of the kind shown in United Kingdom patent specificationNo. 1,429,181. Briefly, in this device at least one of the runners has apawl mounted on it to which the cable is connected either directly orthrough an intermediate mechanism such as a spring biased lever. Thepawl is normally held out of frictional engagement with its associatedguide member by tension in the cable but pivots into frictionalengagement with the inside of the guide member when the tension in thecable is released. However, in this device there is an unsatisfactorydelay in the response of the pawl to the loss of tension in thesupporting cable. Consequently, after a cable breaks the platform andits load, if any, can drop vertically an unacceptable distance along theguide column. Further, as the response time is relatively long theacceleration due to gravity may become quite large and so increase thedifficulty of arresting the momentum of the falling platform by means ofthe interengagement of the pawl with the guide column.

In addition, with the prior device, after the pawl has been actuatedinto locking engagement with the inside of the guide column, the pawlcannot be unlocked except by raising the platform and the body of thelocking device. However, lifts of this kind are fitted with a stopmember at or adjacent the upper end of the guide column against whichthe upper end of the corresponding runner assembly abuts in the fullyraised position of the platform. Thus, if the pawl is locked to thecolumn at a time when the platform is in or adjacent to the fully raisedposition the device cannot be reset except by at least partiallydismantling parts of the apparatus. Accordingly, the prior devicerequires a second auxiliary stop device in the upper part of the columnbeneath the platform stop which must be specially adjusted relative tothe pawl within the column.

Further, taking into account the limited clearance available within aguide column for mounting the parts of the safety device and the rangeof sizes of cable end fittings to be connected to the safety device, theprior device is not practically adaptable to use with heavy-duty largercapacity lifts.

SUMMARY OF THE INVENTION

The foregoing and other disadvantages of the prior device are solved bythe present invention.

The invention comprises a lift with a normally inactive brake assemblythat is interposed between the upper end of a runner and the lower endof a wire rope or the like from which the runner assembly and itsassociated platform is normally suspended. The brake assembly includes abody comprising a spaced pair of plates between which an adaptor link ismounted, at the lower end of the body, for interconnecting the brakeassembly to the upper end of the runner assembly. Spaced from the upperend of the body a terminal block is affixed between the plates to anchorthe lower end of a length of leaf chain whose upper end is anchored toanother terminal block secured to the bottom of a bracket or clevis theupper end of which is connected to an end fitting of the suspension ropeor cable. The leaf chain is sufficiently long such that theinterconnection between the clevis and wire rope end fitting is clear ofthe body of the brake assembly. As a result, the only limitation on thecross-sectional girth of the cable end fitting interconnection is thespace available within the guide column in which the apparatus ismounted rather than the girth of the brake assembly body.

While a leaf chain is preferred for interconnecting the wire rope andbrake assembly, a variety of other forms of flexible element may beemployed for this purpose. In any event, the particular form of flexibleelement employed should be one of high tensile strength which is notsubject to appreciable elongation when subjected to tension but, at thesame time, is laterally flexible or deflectable in response to anysignificant relaxation of tension within itself, such as may occur upona partial or total failure of the wire rope from which the liftapparatus is suspended.

The upper end of the brake body, externally adjacent one side of theplates, pivotally mounts the root ends of a pair of cam pawls on acommon shaft. The upper edges of the pair of pawls are rigidlyinterconnected by a spaced pair of connecting pins between which thelength of leaf chain passes. On the opposite side of the leaf chain fromthe pawl mounting shaft, the cam edges of the pawls are each formed witha spaced series of teeth. The arrangement is such that when the leafchain is in tension it engages one of the cam pins to maintain the pairof pawls in a retracted condition wherein the cam teeth are held out ofengagement with the inside of the guide column. Conversely, upon anysignificant relaxation of tension in the leaf chain, a torsion springfitted to the pawl shaft drives the other cam pin to rotate the pawls todrive their teeth into braking engagement with the inside of the guidecolumn.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a rail type vehicle lift gateincorporating the invention;

FIG. 2 is a side elevational view of a presently preferred embodiment ofthe invention, partly in section, showing the brake in an inactivecondition;

FIG. 3 is a transverse sectional view taken on the line 3--3 of FIG. 2;

FIG. 4 is a rear side elevational view taken on the line 4--4 of FIG. 2;

FIG. 5 is a partial sectional view taken on the line 5--5 of FIG. 2; and

FIG. 6 is a view similar to FIG. 2 but showing the parts in an activatedcondition of the safety brake.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Before explaining the invention in detail, it should be understood thatit is not limited in its application to vehicle lifts nor to the detailsof construction and the arrangement of the components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of application to rail type lifts other than vehicle lift gatesand is also capable of being practiced and carried out in various ways.It should further be understood that the phraseology and terminologyemployed herein is for the purposes of description and should not beregarded as limiting.

Referring to FIG. 1, the tail gate opening of the body of a truck 10 isshown as fitted with a rail type lift gate indicated generally by thenumeral 12. The lift gate 12 schematically represents a large platformself-folding platform lift such as is shown in U.S. Pat. No. Re. 31,157to illustrate application of the invention to heavy duty use, e.g., 6000pound lifts.

Briefly, the illustrated lift comprises a frame having an opposite pairof vertically upstanding members 14 rigidly interconnected adjacenttheir lower ends by a crossbox frame member 16. The rear (with respectto the front end of the vehicle) face of each frame member 14 is fittedwith a fixed guide column 18 the rear face of which is formed throughoutwith a vertically extending median slot 20. A vertically elongate runnerassembly 22 is vertically movably fitted to the rear face of each guidecolumn 18 with the lower ends of the pair of runner assemblies pivotallysupporting a two-piece foldable platform 24. In the case of a heavy dutylift gate, e.g. 6,000 pound capacity, the platform 24, when in theextended horizontal position, is supported at its outer end by means ofan elongate flexible member 26, e.g., a chain, that extends from theouter end of the outer platform section to the upper end of thecorresponding runner assembly 22. More specifically, as best seen inFIGS. 2 and 3 the upper end of each runner assembly 22 is fitted with anintegral pair of spaced apart runner lugs 28 between which the outer endof an adaptor link 30 is received and held in place by a fastening means32. The outer end of the adaptor link is formed with a hole 34 forinterconnection to the upper end of the flexible element or chain 26.

As is schematically indicated in FIG. 1, the upper end of each framemember 14 mounts a pulley or sheave 40 around which a running bight of awire rope or other flexible member 42 is trained. As is shown in FIG. 2,one end portion of the rope 42 descends into the corresponding guidecolumn 18 to terminate in an end fitting 44. The end fitting 44, inturn, is interconnected to one end of a safety brake assembly 46 whoselower end is interconnected, by means of the adaptor link 30, to theupper end of the platform carrying runner assembly 22.

While the runners could take the form of members telescoped within theguide columns, the external mounting depicted in the drawings ispreferred. Thus, referring to FIG. 2, adjacent its upper end each runnerassembly, on its forward face and along its longitudinal centerline, isfitted with a rigid arm 50 that passes with clearance through the guidecolumn slot 20 to mount on opposite sides thereof a pair of rollers 52.A similar arm 50 and corresponding rollers 52 are also fitted adjacentthe lower end of each runner assembly and the vertically spaced apartpairs of rollers are adapted for rolling engagement with interior wallsurfaces of the guide column 18.

The lift apparatus includes a power means (not shown) to effect raisingand lowering of the runner assemblies 22 and load platform 24 betweenground level and the bed height of the truck. A variety of such powermeans are well-known in the art. For example, in the case of the liftillustrated in FIG. 1 it is contemplated that each wire rope 42, afterascending within the guide column 18 and passing over the sheave 40,descends within the corresponding hollow frame member 14 and, by meansof appropriate pulleys or sheaves, enters into the box frame member 16wherein its other end is interconnected to a unidirectionally actinghydraulic cylinder. The power cylinder is interconnected in a hydrauliccircuit including a pump, the hydraulic system being such that when thepump is energized it pressurizes that side of the cylinder piston whichwill effect raising of the platform. Conversely, when it is desired tolower the platform from bed height the pump remains deenergized while avalve control is actuated to bleed the cylinder to allow the force ofgravity to lower the platform.

The brake assembly 46 comprises a body, designated generally by thenumeral 60, comprising a laterally spaced apart pair of identical anchorplates 62. As seen in FIG. 2, the plates 62 are essentially rectangularin plan form and may be held in assembled rigidly spaced apart andcongruent alignment by a means of a gusset plate 64 having its oppositevertical edges welded to forward edges of the plates 62. The body 60 isfurther rigidified by means of a terminal block 66 fitted between thepair of plates 62 and having a pair of opposite ends welded to theplates as shown in FIG. 4. Adjacent their lower ends and on their insideconfronting faces, the pair of plates 62 have a pair of coaxiallyaligned reinforcing washers 68 welded thereto in coaxial alignment witha pair of holes through the plates 62 in order to mount a headed stud 70that is mounted in the aligned holes of the plates and washers 68. Asshown in FIG. 4, opposite ends of the stud 70 extend beyond the outsidesurfaces of the plates 62 and the stud 70 is threaded at one end toreceive a retaining nut 72 as indicated in FIG. 2. The adaptor link 30is formed with a shank 74 projecting forwardly relative to the runnerassembly 22 to pass freely with clearance through the slot 20 of thecorresponding guide column. The inner end of the shank 74 is formed withthe hole that is journaled on the stud bolt 70 in the space between theconfronting pair of reinforcing washers 68.

The terminal block 66 serves to interconnect the body 60 to the lowerend of a short length of flexible element or leaf chain 80 thatprotrudes upwardly beyond the upper end of the body 60. The term"flexible element" means a member such as a length of leaf chain, linkchain, wire rope or the like of great tensile strength but which readilyflexes in response to a lateral force when tension forces on it arereleased. The other end of the chain 80 is interconnected to an upperterminal block 82 which, in effect, is the base of a clevis comprising aspaced pair of parallel arms 84 having their lower ends secured, as bywelding, to opposite sides of the terminal block 82. The arms 84 areformed with coaxially aligned holes for seating an eye bolt 86 whichpasses through the eye of an enlarged head 88 of the wire rope endfitting 44 which has an elongated shank 89 within which the wire rope 42is anchored.

The leaf chain 80 is of a commercially available plate and pinconstruction. More specifically, the chain comprises a plurality offigure-8 shaped parallel plates 90 and parallel pins 92 interconnectedas alternating sets of articulating link plates and pin link plates.Thus, in FIG. 4 the body plates 62, adjacent their upper ends, have theopposite ends of an anchor pin 102 affixed thereto, the pin passingthrough the aligned openings of a set of three of the plates 90 whichcomprise a pin link of the leaf chain. The other ends of the set ofthree plates 90 anchor a connecting pin 92 which also pivotally secureends of four plates 90 comprising an articulating link. This arrangementof alternating pin links and articulating links is repeated for adesired length of leaf chain 80. The upper end of the chain is securedby a cotter pin secured anchor pin 104 in the upper terminal block 82.

Referring to FIG. 2, the spaced apart body plates 62 are formed in theirupper forward corners with coaxially aligned bores to journal a shaft110. As is best seen in FIG. 5, the opposite ends of the shaft 110extend outwardly beyond the pair of plates 62 and pass through bores 112formed in the root ends of an identical pair of cam pawls 114. Oppositeends of the shaft 110 are fitted with snap rings 116 to retain the pawls114 against lateral displacement relative to the body 60. As shown inFIG. 3, the pair of pawls 114 are thus disposed on opposite sides of theleaf chain on a pivot axis that is parallel to the pivot axes of thelinks of leaf chain 80 and each pawl is disposed in alignment with oneof a pair of flanges 118 defined in the rear wall of the guide column 18by the guide slot 20.

The pawls 114 are essentially triangular in configuration and are heldtogether as a unitary assembly by a pair of connecting pins 120 and 122welded or otherwise secured to the upper edges of the pair of pawls. Theconnecting pins 120, 122 are disposed on opposite sides of the leafchain 80. A torsion spring 124 is wound around the pawl support shaft110 and terminates at one end 126 in a leg that bears against the insideof the gusset 64 of the body 60. The other end of the spring 124terminates in a leg 128 extending into the space between the leaf chain80 and the forward connecting pin 122 against which it bears. The pairof cam pawls 114 are thus biased to pivot around the shaft 110 in aclockwise direction as viewed in FIG. 2.

Each of the pawls 114 has a cam edge 130 whose radius relative to theaxis of the shaft 110 increases downwardly. While the cam edge 130 couldtake the form of a friction brake shoe, preferably it is defined by thecrests of a space apart series of hardened teeth 132, each of which isformed with asymmetrical flanks adapted for unidirectional bitingengagement with the inside surface of one of the flanges 118 of theguide column 18 when the brake is actuated. More particularly, theradius of the cam edge 130 is such that when it occupies a retractedposition as shown in FIG. 2, the radius at the upper end of a pawl 114is sufficiently short that the teeth 132 are spaced away from thecorresponding flange 118. The cam radius increases downwardly to alength in excess of that required to jam the side of the body 60opposite the cam teeth 132, i.e. the gusset plate 64, against the insidesurface of the forward wall of the guide column 18 as the pawls 114rotate clockwise into an extended position in which the teeth 132 biteinto the inside surface of one of the flanges 118.

Assuming no break in the wire rope 42, the load suspended dictates thestraight and tensioned condition of the leaf chain 80 illustrated inFIG. 2. Since the force of the torsion spring 124 is insufficient tocause deflection of the chain by means of the connecting or contact pin120, the pair of pawls 114 are thus maintained in the retractedcondition of FIG. 2, permitting normal use of the lift gate in raisingand lowering loads. However, any significant loss of tension in the wirerope 42 is substantially immediately reflected as buckling of the leafchain 80 whereupon the torsion spring 124 pivots the pawls 114 into theextended position depicted in FIG. 6. The connecting pin 120 is thusenabled to deflect the leaf chain 80, the hardened cam teeth 132 thusdig into the relatively soft inside surface of the flange 118, and thegusset plate 64 of the body 60 is jammed against the forward wall of theguide tube 18. The mechanism works somewhat like an internal expandingbrake that is very sensitive to significant losses of tension in thewire rope 42 to lock the load platform and its load more quickly afterfailure of the suspension device than has heretofore been possible.

Assuming the locked condition of FIG. 6, it will be seen that the brakeassembly 46 can be unlocked merely by applying an upward force on theclevis mechanism 82, 84, without, at the same time, having to raise arunner assembly 22 or the brake body 60. Such upward force could beapplied, for example, by lifting on some portion of the wire rope 42above the cable end fitting 88. This ability to unlock the brakemechanism 46 is important in a situation where failure of the rope 42has occurred at or closely adjacent to the upper limit of travel of therunners 22. More specifically, as schematically shown in FIG. 1, inlifts of this kind each of the guide columns 18 is fitted at or adjacentits upper end with an up-stop means 140. This may take the form of anexternal bracket plate clamped to a member within the guide column 18which projects downward sufficiently to engage some portion of therunner assembly, for example, the shank 74 of the adaptor link 30, tonormally arrest the load platform 24 at the level of the bed height ofthe vehicle.

Preferably, the hoisting ropes or cables of both guide columns 18 arefitted with a brake assembly 46 in order to divide the load to bearrested in the event of a failure of either or both cables. Also, thebrake assembly 46 may be incorporated as part of the original liftequipment or it may be retro-fitted to existing lifts or lift gates.

I claim:
 1. A safety brake for mounting within a guide column of a lift,said safety brake comprising:a body; a means at the lower end of saidbody for connecting a runner to said body; a first terminal blockaffixed to said body at a location spaced below the upper end of saidbody; a length of a flexible element having a lower end secured to saidfirst terminal block; a second terminal block secured to an upper end ofsaid length of flexible element, whereby said second terminal block isdisplaceable relative to said body in substantially the direction of theaxis of said flexible element upon a lateral deflection of said flexibleelement; a means on said second terminal block for interconnecting saidsecond terminal block to a powered vertically reciprocable means forsuspending said flexible element and said body from said second terminalblock, whereby said flexible element is gravitationally biased into astraightened condition by the weight of said body and the weight of arunner and a load to be carried by the runner; a pawl mounted on saidbody for movement of said pawl relative to said body between retractedand extended positions of said pawl; means operatively connected betweensaid body and said pawl for biasing said pawl into movement from saidretracted position into said extended position; and contact meanscarried by said pawl in a position so located that said flexibleelement, when in tension, normally holds said pawl in said retractedposition against the force of said pawl biasing means and whereby saidcontact means, upon relaxation of tension in said flexible element, iscarried by the force of said pawl biasing means in the direction inwhich said flexible element is laterally deflected as said pawl movesinto said extended position; said pawl and said body expanding intocontact with opposite surfaces of the guide column in which said safetybrake is mounted upon movement of said pawl into said extended position.2. A safety brake as in claim 1 wherein:said flexible element comprisesa leaf chain.
 3. A safety brake as in claim 2 wherein:said pawl ispivotally mounted on said body on a pivot axis that is parallel to thepivot axes of the links of said leaf chain.
 4. A safety brake as inclaim 3 wherein:said pivot axis of said pawl is located on one side ofsaid flexible element and between opposite ends of said flexibleelement; said contact means carried by said pawl being disposed on theopposite side of said flexible element from said pivot axis of saidpawl.
 5. A safety brake as in claim 1 wherein:said flexible elementprotrudes beyond said upper end of said body whereby said secondterminal block and said means on said second terminal block forinterconnecting said terminal block to a powered vertically reciprocablemeans are disposed outside of the confines of said body.
 6. A safetybrake as in claim 5 wherein:said pawl is pivotally mounted on said bodyon a pivot axis located on one side of said flexible element and betweenopposite ends of said flexible element; said contact means carried bysaid pawl being disposed on the opposite side of said flexible elementfrom said pivot axis of said pawl.
 7. A safety brake for mounting withina guide column of a lift, said safety brake comprising:a body comprisinga parallel pair of plates, said body being adapted for connection to arunner; a first terminal block having opposite ends rigidly secured tosaid pair of plates; a length of a leaf chain having a lower end securedto said first terminal lock, said leaf chain having the pivotal axes ofthe links of said leaf chain oriented normal to the planes of saidplates; a second terminal block secured to an upper end of said leafchain, said second terminal block comprising part of a clevis forinterconnecting said second terminal block to a powered verticallyreciprocable means for suspending said leaf chain and said body fromsaid second terminal block whereby said leaf chain is gravitationallybiased into a straightened condition by the weight of said body and theweight of a runner and a load to be carried by the runner; a pair of campawls having root ends pivotally mounted on said body on a pivot axisthat is disposed on one side of said leaf chain, said pivot axis of saidpair of cam pawls being disposed parallel to the pivot axes of the linksof said leaf chain, said pair of pawls each having a cam edge that isswingable between retracted and extended positions relative to saidbody; spring means operatively connected between said body and said pairof cam pawls for biasing said pawls into movement from said retractedposition into said extended position; a contact pin mounted on said pairof cam pawls in a position for contacting that side of said leaf chainopposite to said pivot axis of said pair of cam pawls whereby said leafchain, when in tension, holds said pawls in said retracted positionagainst the force of said spring means and whereby said contact pin,upon relaxation of tension in said leaf chain, is driven by the force ofsaid spring means in a direction to laterally deflect said leaf chain assaid pawls move into said extended position, said pawls and said bodyexpanding into contact with opposite surfaces of the guide column inwhich said safety brake is mounted upon movement of said pawls into saidextended position.
 8. A safety brake as in claim 7 wherein:said cam edgeof each of said pawls is formed with a series of spaced teeth adaptedfor unidirectional braking engagement with a surface of the guidecolumn.
 9. A safety brake as in claim 7 wherein:said pair of pawls aredisposed in planes parallel to and outside of said pair of parallelplates.
 10. A safety brake as in claim 9 wherein:said pair of pawls haveupper edges disposed above upper ends of said pair of plates and saidcontact pin is affixed to said upper edges of said pawls on an axisparallel to pivot axes of the links of said leaf chain.
 11. A safetybrake as in claim 10 wherein:said spring means comprises a torsionspring, a shaft on said body on which said pawls are pivotally mounted,and a connecting pin fastened at opposite ends to said upper edges ofsaid pawls, said torsion spring being mounted on said shaft and havingone end biased against said connecting pin, said connecting pin and saidcontact pin being disposed on opposite sides of said leaf chain.
 12. Afail safe lift apparatus comprising:a frame; an upright hollow guidecolumn secured to said frame; a runner means operatively connected tosaid guide column for vertical reciprocation therealong, said runnermeans having a means for interconnecting a load platform thereto; alength of a flexible element having a lower end secured to said runnermeans; a means for securing the upper end of said flexible element to ameans for vertically reciprocating said flexible element and said runnermeans along said guide column, whereby said flexible element and saidrunner are suspended from said securing means with said flexible elementbeing gravitationally biased into straightened condition by the weightof said runner means and a load to be carried by the platform, saidflexible element comprising the sole means for supporting the weight ofsaid runner means and a load to be carried by the platform; and a meansoperatively interconnected between said runner means and said flexibleelement for locking said runner means against descending relative tosaid guide column in response to a lateral deflection in said flexibleelement induced by a loss of tension in said flexible element.
 13. Afail safe lift apparatus as in claim 12 in which:said locking meanscomprises a spring biased pawl that is normally held out of lockingengagement with said guide column by contact between said pawl and oneside of said flexible element whenever said flexible element is undersufficient tension to be maintained in a straightened condition.
 14. Afail safe lift apparatus as in claim 12 in which:said length of aflexible element comprises a leaf chain.